The present invention relates to a misregister amount detection method and apparatus which sense printed register marks of colors and obtain the positional shift of the sensed mark of each color as a misregister amount.
In a conventional offset printing press, a printing plate with a printed image is attached to the outer surface of a plate cylinder. Ink stored in an ink fountain is supplied to the printing plate through ink rollers. The ink supplied to the printing plate is transferred to a blanket cylinder which rotates in press contact with the plate cylinder so that printing is executed for a printing paper sheet passing between the blanket cylinder and an impression cylinder. In multicolor printing, a plurality of printing units each having an ink fountain, ink rollers, plate cylinder, blanket cylinder, and impression cylinder are prepared for the respective colors. The printing units sequentially print the colors on a conveyed printing paper sheet.
As shown in FIG. 28, each printing unit comprises an inking device (inker) corresponding to each color. Referring to FIG. 28, reference numeral 1 denotes an ink fountain which stores ink 2; 3, an ink fountain roller; 4-1 to 4-n, a plurality of ink fountain keys which are juxtaposed in the axial direction of the ink fountain roller 3; 5, an ink ductor roller; 6, ink rollers; and 7, a plate cylinder to which a printing plate 8 is attached. An image is printed on the printing plate 8 mounted on the outer surface of the plate cylinder 7.
In each printing unit, the amount of ink supplied from the ink fountain 1 to the ink fountain roller 3 is adjusted by adjusting the opening ratios of the ink fountain keys 4-1 to 4-n. Simultaneously, the amount of ink supplied from the ink fountain roller 3 to the printing plate 8 through the ink rollers 6 is adjusted by adjusting the feed rate of the ink fountain roller 3. The ink supplied to the printing plate 8 is transferred to the blanket cylinder (not shown) and printed on a printing paper sheet.
In a multicolor printing press having a plurality of printing units, the printing plate 8 wound around the plate cylinder 7 of each color printing unit may be shifted from an appropriate position. In this case, color images of a finished printing product may have a positional shift (misregister) in the circumferential direction [the printing direction in the printing press (the direction of travel of a printing product)] or lateral direction (a direction perpendicular to the circumferential direction).
To solve this, in Japanese Patent Laid-Open No. 62-99149 (reference 1), a portion where cross-shaped register marks are printed as register marks of the respective colors is zoomed and sensed. The position of each color register mark contained in the sensed image is calculated. The misregister amount between the colors and the position correction amount of the plate cylinder are calculated from the calculated register mark positions. The position of the plate cylinder is automatically corrected on the basis of the calculation result.
FIG. 29 shows the schematic arrangement of the automatic registration apparatus of a printing press described in reference 1. Referring to FIG. 29, reference numeral 11 denotes a camera (color camera); 12, a processing unit; 13, an operation panel; 14, a motor driving circuit; 15, a plurality of motors to drive the register adjustment elements of the printing press; and 16, a plurality of potentiometers attached to the motors 15. The camera 11 zooms and senses an image portion X where four register marks TC, TM, TY, and TK of cyan (C), magenta (M), yellow (Y), and black (K) are printed. R image data with a red component R, G image data with a green component G, and B image data with a blue component B of the register marks TC, TM, TY, and TK are sent to the processing unit 12.
The processing unit 12 stores the R image data from the camera 11 in an R frame memory (not shown), the G image data in a G frame memory (not shown), and the B image data in a B frame memory (not shown). The R, G, and B image data stored in the R, G, and B frame memories are converted into C, M, Y, and K image data and stored in C, M, Y, and K frame memories. The R, G, and B image data are converted into C, M, Y, and K image data in the following way.
(1) The complement of the luminance level (pixel value) of each pixel of the R, G, and B image data is set to the density level (pixel value) of each pixel of the C, M, and Y image data and stored in the C, M, and Y frame memories.
(2) The density level of each pixel of the C, M, and Y image data is checked. For a pixel having a density of a predetermined level or more in all the C, M, and Y colors, the average density level of the C, M, and Y colors is obtained. The average density level is stored as the pixel value of a corresponding pixel in the K frame memory. In this case, the density levels of corresponding pixels in the C, M, and Y frame memories are rewritten to 0.
(3) The luminance level of each pixel of the R image data is checked. A pixel lower than a predetermined level (a pixel having a high C density level) is determined as a portion where C and K overlap. The complement of the luminance level is converted into a density level. The density level is multiplied by a predetermined constant to remove the increase in density by K. The density level of a corresponding pixel in the C frame memory is rewritten.
(4) The luminance level of each pixel of the G image data is checked. A pixel lower than a predetermined level (a pixel having a high M density level) is determined as a portion where M and K overlap. The complement of the luminance level is converted into a density level. The density level is multiplied by a predetermined constant to remove the increase in density by K. The density level of a corresponding pixel in the M frame memory is rewritten.
(5) The luminance level of each pixel of the B image data is checked. A pixel lower than a predetermined level (a pixel having a high Y density level) is determined as a portion where Y and K overlap. The complement of the luminance level is converted into a density level. The density level is multiplied by a predetermined constant to remove the increase in density by K. The density level of a corresponding pixel in the Y frame memory is rewritten.
With the above-described process, the R, G, and B image data are converted into C, M, Y, and K image data and stored in the C, M, Y, and K frame memories.
(6) To check which pixel of the C, M, Y, and K image data has an image portion, the density level of each pixel stored in the C, M, Y, and K frame memories is binarized with a predetermined slice level.
(7) In two directions (X and Y directions) perpendicular to each other in the C image data, the number of pixels determined to have an image is added. The sum of images in the Y direction is plotted for each pixel position in the X direction to obtain a first cumulative curve. The sum of images in the X direction is plotted for each pixel position in the Y direction to obtain a second cumulative curve. The center of the peak of the first cumulative curve is obtained as the X-coordinate of the center of the register mark TC. The center of the peak of the second cumulative curve is obtained as the Y-coordinate of the center of the register mark TC.
(8) In two directions (X and Y directions) perpendicular to each other in the M image data, the number of pixels determined to have an image is added. The sum of images in the Y direction is plotted for each pixel position in the X direction to obtain a first cumulative curve. The sum of images in the X direction is plotted for each pixel position in the Y direction to obtain a second cumulative curve. The center of the peak of the first cumulative curve is obtained as the X-coordinate of the center of the register mark TM. The center of the peak of the second cumulative curve is obtained as the Y-coordinate of the center of the register mark TM.
(9) In two directions (X and Y directions) perpendicular to each other in the Y image data, the number of pixels determined to have an image is added. The sum of images in the Y direction is plotted for each pixel position in the X direction to obtain a first cumulative curve. The sum of images in the X direction is plotted for each pixel position in the Y direction to obtain a second cumulative curve. The center of the peak of the first cumulative curve is obtained as the X-coordinate of the center of the register mark TY. The center of the peak of the second cumulative curve is obtained as the Y-coordinate of the center of the register mark TY.
(10) In two directions (X and Y directions) perpendicular to each other in the K image data, the number of pixels determined to have an image is added. The sum of images in the Y direction is plotted for each pixel position in the X direction to obtain a first cumulative curve. The sum of images in the X direction is plotted for each pixel position in the Y direction to obtain a second cumulative curve. The center of the peak of the first cumulative curve is obtained as the X-coordinate of the center of the register mark TK. The center of the peak of the second cumulative curve is obtained as the Y-coordinate of the center of the register mark TK.
(11) The misregister amounts in the circumferential and lateral directions between the colors are obtained from the obtained central coordinates of the register marks TC, TM, TY, and TK. The motors 15 are so driven as to nullify the misregister amounts while receiving feedback of position data from the potentiometers 16, thereby correcting the position of the plate cylinder 7 in each color printing unit.
However, in the conventional misregister amount detection method described in reference 1, in generating the K image data from the C, M, and Y image data in the step (2), a pixel having a density of a predetermined level or more in all the C, M, and Y colors is determined as K. However, the C, M, Y, and K densities change between actual printing products. Hence, if K pixels are determined in accordance with a threshold value of predetermined level, it makes a difference between printing products.
In the step (3), a pixel in the R image data whose luminance level is lower than a predetermined level is determined as a pixel where C and K overlap. In the step (4), a pixel in the G image data whose luminance level is lower than a predetermined level is determined as a pixel where M and K overlap. In the step (5), a pixel in the B image data whose luminance level is lower than a predetermined level is determined as a pixel where Y and K overlap. However, the C, M, Y, and K densities, i.e., the R, G, and B luminance levels change between actual printing products. Hence, if pixels overlapping K are determined in accordance with a threshold value of predetermined level, it makes a difference between printing products.
In the step (3), the complement of the luminance level of a pixel where C and K overlap is multiplied by a predetermined constant, thereby obtaining the C density level excluding the increase in density by K. In the step (4), the complement of the luminance level of a pixel where M and K overlap is multiplied by a predetermined constant, thereby obtaining the M density level excluding the increase in density by K. In the step (5), the complement of the luminance level of a pixel where Y and K overlap is multiplied by a predetermined constant, thereby obtaining the Y density level excluding the increase in density by K. However, the C, M, Y, and K densities change between actual printing products. Hence, if the density level excluding the increase in density by K is obtained by multiplication of a predetermined constant, it makes a difference between printing products.
In the step (6), to check which pixel has an image, the density level of each pixel stored in the C, M, Y, and K frame memories is binarized with a predetermined slice level. However, the C, M, Y, and K densities change between actual printing products. Hence, if the density level is binarized with a predetermined slice level, it makes a difference between printing products.
As described above, in the conventional misregister amount detection method of reference 1, K pixels are determined in accordance with a threshold value of predetermined level, or pixels overlapping K are determined in accordance with a threshold value of predetermined level, although the C, M, Y, and K densities change between actual printing products. In addition, the density level excluding the increase in density by K is obtained by multiplication of a predetermined constant, or the density level is binarized with a predetermined slice level. For this reason, a difference is generated between printing products. A detection error occurs in the central coordinates of the register marks of the respective colors. Hence, the misregister amount between the colors cannot accurately be obtained.